<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>模拟圆角效果</title>
</head>
<body>
   <h3>模拟圆角效果：</h3>
  <div class="top"></div>
  <div class="cont">中间部分</div>
  <div class="bottom"></div>
  <h3>分解图：</h3>
  <div class="top top1">border-bottom</div>
  <div class="cont cont1">中间部分</div>
  <div class="bottom bottom1">border-top</div>
  <style>
    div{
      width: 200px;
      border: 3px solid transparent;
    }
    .top{
      border-bottom-color: red;
    }
    .cont{
      height: 56px;
      background: red;
      border-color: red;
    }
    .bottom{
      border-top-color: red;
    }
    .cont1{
      border-width: 30px;
    }
    .top1{
      border-width: 30px;
      border-bottom-color: rgb(83, 0, 0);
    }
    .bottom1{
      border-width: 30px;
      border-top-color: rgb(83, 0, 0);
    }
  </style>
</body>
</html>